Method of manufacturing enhanced spin-valve sensor with engineered overlayer

ABSTRACT

A method for making an enhanced spin valve sensor with engineered overlayer for sensing magnetically recorded information on a data storage medium. The method includes forming a ferromagnetic free layer and a ferromagnetic pinned layer sandwiching an electrically conductive spacer layer. An overlayer is formed on the free layer and adapted to decrease free layer magnetic thickness without reducing physical thickness.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetoresistive sensors for readingmagnetically recorded information from data storage media, andparticularly to spin valve read sensors for direct access storage device(DASD) systems.

2. Description of the Prior Art

By way of background, spin valve sensors, also known as giantmagnetoresistive (GMR) sensors, are commonly used in read heads formagnetic media-based DASD systems, such as disk drives. A spin valvesensor is a magneto-electrical device that produces a variable voltageoutput in response to magnetic field fluctuations on an adjacentmagnetic storage medium. As illustrated in FIG. 1, a conventional spinvalve device is formed by first and second ferromagnetic layers,hereinafter referred to as a “pinned” layer and a “free” layer,separated by an electrically conductive spacer layer. In a disk drive,these layers are oriented so that one edge of the layer stack faces anadjacent disk surface, in a cross-track direction, and so that the layerplanes of the stack are perpendicular to the disk surface. The magneticmoment (M₁) of the pinned layer is oriented at an angle θ₁ that isperpendicular to the disk surface (i.e., θ₁=90°). It is sometimesreferred to as the “transverse” magnetic moment of the sensor. Themagnetic moment M₁ is substantially pinned so that it will not rotateunder the influence of the disk's magnetic domains. Pinning is typicallyachieved by way of exchange coupling using an adjacent antiferromagneticpinning layer. The magnetic moment (M₂) of the free layer has a zerobias point orientation θ₂ that is parallel to the disk surface (i.e.,θ₂=0°). It is sometimes referred to as the “longitudinal” magneticmoment of the sensor. The magnetic moment M₂ is free to rotate inpositive and negative directions relative to the zero bias pointposition when influenced by positive and negative magnetic domainsrecorded on the disk surface. In a digital recording scheme, thepositive and negative magnetic domains correspond to digital “1s” and“0s.” The zero bias point is the position of the free layer magneticmoment M₂ when the sensor is in a quiescent state and no externalmagnetic fields are present.

Electrical leads are positioned to make electrical contact with thepinned, free and spacer layers. In a CIP (Current-In-Plane) spin valvesensor, as shown in FIG. 1, the leads are arranged so that electricalcurrent passes through the sensor stack in a cross-track directionparallel to the layer planes of the stack. When a sense current isapplied by the leads, a readback signal is generated in the driveprocessing circuitry which is a function of the resistance changes thatresult when the free layer magnetic moment M₂ rotates relative to thepinned layer magnetic moment M₁ under the influence of the recordedmagnetic domains. These resistance changes are due toincreases/decreases in the spin-dependent scattering of electrons at theinterfaces of the spacer layer and the free and pinned layers as thefree layer's magnetic moment M₂ rotates relative to the magnetic momentM₁ of the pinned layer. Resistance is lowest when the free and pinnedlayer magnetic moments are parallel to each other (i.e., θ₂=90°) andhighest when the magnetic moments are antiparallel (i.e., θ₂=−90°). Theapplicable relationship is as follows:

-   -   ΔR∝cos(θ₁−θ₂)∝sin θ₂. The ΔR resistance changes cause potential        differences that are processed as read signals.

It is important that a spin valve sensor exhibits high GMR effect ratio(i.e., a high ratio of change in resistance to the resistance of thesensor as a function of an applied magnetic field) in order to providemaximum sensitivity. It is likewise desirable to construct the freelayer so that it exhibits controlled negative magnetostriction for highstability.

As the areal density in magnetic recording increases, it is necessary toreduce the magnetic thickness of both the recording medium and the freelayer of the sensor. The magnetic thickness of a material is given bythe product of the remanent magnetic moment density (Mr) and physicalthickness (t) of the material, and is commonly expressed as Mr*t. Theconventional approach to decreasing the free layer magnetic thickness ina spin valve sensor is to decrease the free layer's physical thickness,e.g., from 30 Å to 25 Å or below. Unfortunately, reducing the freelayer's physical thickness tends to decrease sensor sensitivity byreducing its GMR ratio and causing free layer magnetostriction to becomemore positive.

One approach to improving the performance of spin valve sensors withthin free layers is a “spin filter” design, in which a thin layer ofhighly electrically conductive and non-magnetic material, typicallycopper (Cu), is inserted between the sensor free layer and its(preferably oxide) cap layer. With this design, however, the spin valvesheet resistance undesirably decreases.

Accordingly, a need exists for a GMR sensor configuration whereindecreased free layer magnetic thickness is achieved in order toaccommodate increased areal data densities while maintaining high sensorGMR ratio and controlled negative magnetostriction in the free layer.What is required in particular is a GMR sensor having a free layer withdecreased magnetic thickness and improved sensitivity without having todecrease free layer physical thickness and thereby negatively impactsensor GMR ratio and free layer magnetostriction.

SUMMARY OF THE INVENTION

The foregoing problems are solved and an advance in the art is obtainedby a novel GMR sensor for sensing magnetically recorded information on adata storage medium, together with fabrication methods therefor. Thesensor includes a ferromagnetic free layer and a ferromagnetic pinnedlayer sandwiching an electrically conductive spacer layer. An engineeredoverlayer is formed on the free layer to decrease free layer magneticthickness without reducing physical thickness, which allows high sensorGMR ratio and more negative magnetostriction in the free layer.

In exemplary embodiments of the invention, the overlayer is aprotective, non-electrically conducting structure that defines a sharpnon-diffuse interface with the free layer, promotes elastic scatteringor spin-dependent reflection of sense current electrons with consequentmaintenance of GMR effect, and minimizes sense current shunting awayfrom the ferromagnetic (and spacer) layers. The overlayer preferablycomprises a metal oxide, such as a material selected from the groupconsisting of aluminum oxide, tantalum oxide or other transition metaloxides such as zirconium oxide, titanium oxide, hafnium oxide, etc., andmagnesium oxide. The overlayer can be formed by any suitable physicalvapor deposition process, such as ion beam deposition or magnetronsputtering, with oxidation of the metal component of the overlayer beingperformed according to design preferences. The free layer magneticthickness may range from approximately 35–26 Å or below. The desiredmagnetic thickness is preferably achieved while maintaining the sensor'sGMR ratio in a range of approximately 13–15% and the free layer'smagnetostriction in a range of approximately zero to −2×10⁻⁶.

The invention further contemplates methods for fabricating GMR sensorswith an engineered overlayer formed on the free layer, as well asmagnetic heads and disk drives incorporating such sensors.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingDrawing, in which:

FIG. 1 is a perspective view showing a conventional GMR spin valvesensor;

FIG. 2 is a side elevational view showing the interior of a disk driveincorporating a GMR spin valve sensor constructed in accordance with thepresent invention;

FIG. 3 is a plan view of the disk drive of FIG. 2;

FIG. 4 is a plan view of an integrated read/write transducer for use inthe disk drive of FIG. 2;

FIG. 5 is a side elevational view of the transducer of FIG. 4;

FIG. 6 is a cross-sectional view taken along line 6—6 in FIG. 4;

FIG. 7 is an ABS view of the transducer of FIG. 3 taken in the directionof arrows 7—7 in FIG. 6;

FIG. 8 is a detailed ABS view of an exemplary embodiment of a GMR spinvalve sensor constructed in accordance with the invention;

FIG. 9 is a flow diagram showing an exemplary method for fabricating GMRspin valve sensors in accordance with the invention;

FIG. 10 is a graph showing free layer magnetic thickness as a functionof oxidation used during overlayer formation;

FIG. 11 is a graph showing GMR ratio as a function of free layermagnetic thickness; and

FIG. 12 is a graph showing magnetostriction as a function of free layermagnetic thickness.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to the figures (which are not necessarily to scale), whereinlike reference numerals represent like elements in all of the severalviews, FIGS. 2 and 3 illustrate an exemplary disk drive 2 thatincorporates a GMR spin valve sensor having an engineered overlayer inaccordance with the invention. Note that the disk drive 2 is shown ingreatly simplified schematic form, with only those construction detailsthat are necessary for an understanding of the invention beingrepresented. As to these illustrated components, it should be understoodthat all are conventional in nature unless otherwise indicated below.

The disk drive 2 conventionally includes a base casting 4 made fromaluminum or other suitable material. A cover 5 is removably mountedthereto via a hermetic seal (not shown). The base casting 4 supports aconventional spindle drive motor 6 having an associated drive spindle 8.The drive spindle 8 carries a set of disks 10 for high speed rotationtherewith. The disks 10 form a spaced vertically stacked disk platterarrangement. Each disk 10 is conventionally formed from an aluminum orglass substrate with appropriate coatings being applied thereto suchthat at least one, and preferably both, of the upper and lower surfacesof the disks are magnetically encodable and aerodynamically configuredfor high speed interaction with a read/write transducer (describedbelow).

Data access to the disk surfaces is achieved with the aid of an actuator12 that is mounted for rotation about a stationary pivot shaft 14. Theactuator 12 includes a set of rigid actuator arms 16 that respectivelycarry either one or two flexible suspensions 18 (see FIG. 2). Eachsuspension 18 supports a slider 20 and a transducer 22 that arepositioned to interact with an associated disk surface, representing thetransducer's recording medium. The sliders 20 are aerodynamicallydesigned so that when the disks 10 are rotated at operational speed, anair bearing develops between each slider and its associated disksurface. The air bearing is very thin (typically 0.05 μm) so that thetransducers 22 are positioned in close proximity to the recording media.A conventional voice coil motor 24 is provided for pivoting the actuator12. This motion sweeps the actuator arms 16 and their slider-carryingsuspensions 18 generally radially across the respective surfaces of thedisks 10, allowing the transducers 22 to be positioned from oneconcentric data track to another during seek, settle and track followingoperations of the drive 2.

As described in more detail below, each transducer 22 is an integrateddevice that includes a magnetic write head and a GMR spin valve sensorread head constructed in accordance with the invention. Data is readfrom the disks 10 by the read head portion of each transducer 22. Thisdata is processed into readback signals by signal amplification andprocessing circuitry (not shown) that is conventionally located on eachactuator arm 16. The readback signals carry either customer data ortransducer position control information depending on whether the activeread head is reading from a customer data region or a servo region onone of the disks 10. The readback signals are sent to the drivecontroller 25 for conventional processing. Data is recorded on the disks10 by the write head portion of each transducer 22. This data isprovided by write data signals that are generated by the controller 25during data write operations. The write data signals are delivered towhichever write head is actively writing data. The active write headthen records the positive and negative magnetic domains representingdigital information to be stored onto the recording medium.

Turning now to FIGS. 4–7, an exemplary one of the transducers 22 isshown as including a transducer write head portion 26 and a transducerread head portion 28. In FIGS. 4–6, the transducer 22 is shown as beinglapped at 29 to form an air bearing surface (ABS) where the transducermagnetically interacts with the adjacent rotating disk surface. The ABS29 is spaced from the disk surface during drive operations by virtue ofthe above-described air bearing. FIG. 7 depicts the transducer 22 fromthe vantage point of the disk surface, looking toward the ABS 29.

The write head 26 conventionally includes a first insulative layer 30(commonly referred to as “I1”) supporting a second insulative layer 32(commonly referred to as “I2”) that carries plural inductive coil loops34. A third insulative layer 35 (commonly referred to as “I3”) can beformed above the coil loops 34 for planarizing the write head 26 toeliminate ripples in the I2 insulative layer 32 caused by the coilloops. The coil loops 34 inductively drive first and second pole pieces36 and 38 that form the yoke portion of the write head 26. The polepieces 36 and 38 respectively extend from a back gap 39 to pole tips 36a and 38 a located at the ABS 29. An insulative gap layer 40 (commonlyreferred to as “G3) is sandwiched between the pole pieces 36 and 38 toprovide a magnetic write gap at the pole tips 36 a and 38 a. Note thatthe pole piece 36 is commonly referred to as a “P1” pole piece. The polepiece 38 may be referred to as a “P2” or “P3” pole piece depending onhow the pole tip 38 a is formed. It is labeled as “P2” in FIG. 5. Duringdata write operations, electrical current passing through a pair ofelectrical leads E1 and E2 to the coil loops 34 generates a magneticfield that induces a magnetic flux in the P1 and P2 layers 36 and 38. Asshown in FIG. 6, this magnetic flux propagates from the yoke to the poletips 36 a and 38 a, where it fringes across the gap layer 40 at the ABS29. This causes magnetic domains to be formed on an adjacent recordingsurface of one of the disks 10. The orientation of each recordedmagnetic domain is dependent on the magnetization direction of the poletips 36 a and 38 a, which in turn is determined by the direction of theelectrical current passing through the coil loops 34. Reversing thecoil's electrical current reverses the magnetization direction of thepole tips 36 a and 38 a, and consequently reverses the orientation ofthe next recorded magnetic domain. This magnetization reversal processis used to encode data on the recording medium.

The read head 28 lies between insulative gap layers 42 and 44 at the ABS29, where it is influenced by magnetic flux emanating from the adjacentdisk surface. The gap layers 42 and 44 are commonly referred to as “G1”and “G2” gap areas, and are sandwiched between a first magnetic shieldlayer 46 (commonly referred to as an “S1” shield) and second magneticshield layer 48 (commonly referred to as an “S2” shield). In somedesigns, including that of FIG. 5, the S2 shield layer 48 also providesthe P1 pole piece 36. The P1 shield layer 46 is conventionally formedover the slider 20, which is only partially shown in FIGS. 5 and 6 forclarity.

Turning now to FIG. 8, the read head 28 is shown to comprise a GMR spinvalve sensor 50 that is constructed in accordance with a preferredembodiment of the invention. As in the case of FIG. 7, the view of FIG.8 is taken on a plane that is parallel to the ABS 29. The “x” axis inFIG. 8 represents the radial track width direction of a concentric trackon the adjacent disk surface. The “y” axis in FIG. 8 represents thecircumferential centerline direction of a concentric track on the disk.The “z” axis represents the direction pointing perpendicularly into thedisk surface.

It will be seen in FIG. 8 that the sensor 50 has multiple materialslayers that are sandwiched between the S1 and G1 layers 46 and 42 on oneside, and the S2 and G2 layers 48 and 44 on the other side. A pair ofelectrical lead structures 52 and 54 are situated to deliver a sensecurrent “I” to the sensor 50 according to a CIP orientation.

The sensor 50 is implemented as a “bottom-type” spin valve sensor. Itthus begins with a ferromagnetic pinned (P) layer 56 whose magnetizationdirection is fixed perpendicular to the plane of FIG. 8. Although thepinned layer 56 could be self pinned, for example, by forming it withvery high positive magnetostriction and very large compressive stress(according to existing techniques), FIG. 8 shows an implementationwherein the pinned layer 56 is externally pinned by an optionalantiferromagnetic (AFM) pinning layer 58. The pinning layer 58 isdeposited to a suitable thickness on one or more conventional seedlayers that are formed on top of the G1 gap layer 42. The pinning layer58 can be made from platinum-manganese (Pt—Mn), nickel-manganese(Ni—Mn), iridium-manganese (Ir—Mn), or any other suitableantiferromagnetic material that is capable of exchange biasing theferromagnetic material in the pinned layer 56.

The pinned layer 56 can be implemented in conventional fashion as asingle layer ideally having one magnetization direction, or as pluralsub-layers ideally having parallel and anti-parallel magnetizationdirections. FIG. 8 shows an example of the latter configuration, withthe pinned layer 56 being formed by growing a first sublayer 56 a ofcobalt-iron (CoFe), a second sublayer 56 b of ruthenium (Ru), and athird sublayer 56 c of cobalt-iron (CoFe). These sublayers are formed ontop of the pinning layer 58 at suitable thicknesses. The magnetic momentof the first sublayer 56 a is shown by the arrow tail 60 a, which pointsinto the plane of FIG. 8. The magnetic moment of the third sublayer 56 cis shown by the arrowhead 60 b, which points out of the plane of FIG. 8.The magnetic moments 60 a and 60 b are thus antiparallel to each otherand oriented generally perpendicular to the sensing surface (ABS) of thesensor 50.

As stated, the pinned layer 56 will have its magnetic moment fixed byinterfacial exchange coupling with the pinning layer 58. Themagnetization direction(s) of the pinned layer 56 will be sufficientlyfixed by the exchange-biasing pinning layer 58 to prevent rotationthereof in the presence of relatively small external magnetic fields,such as the fields produced by magnetic domains recorded on the adjacentdisk surface.

A spacer layer 62 is formed on top of the pinned layer 56 as a suitablythick deposit of an electrically conductive, non-ferromagnetic material,such as Cu.

The sensor's free layer 64 is formed above the spacer layer 62. The freelayer 64 can be made by covering the spacer layer 62 with a single layerof Co, Co—Fe, Ni—Fe or other suitable ferromagnetic material grown to asuitable thickness. In an alternative configuration, the free layer 64can be formed from multiple layers, such as a bilayer structurecomprising a bottom sublayer of Co—Fe and a top sublayer of Ni—Fe, or atrilayer structure comprising a bottom sublayer of Co—Fe, a middlesublayer of Ni—Fe and a top sublayer of Co—Fe.

The arrow 66 in FIG. 8 shows the preferred zero bias point magnetizationdirection of the free layer 64 when the sensor 50 is in a quiescentstate with no magnetic field incursions from the adjacent disk surface.The magnetization direction 66 is preferably stabilized in suitablefashion, as by incorporating hard biasing regions (not shown) in thelead structures 52 and 54 so as to form a contiguous junction with thesides of the free layer 62. The hard biasing regions can beconventionally formed of ferromagnetic material with relatively highmagnetic coercivity (H_(c)), such as cobalt-chromium-platinum (CoCrPt)and alloys thereof.

An engineered, protective, electrically non-conducting overlayer 68 isformed on the surface of the free layer 64 in order to reduce free layermagnetic thickness while preserving high GMR ratio and negative freelayer magnetostriction. The overlayer 68 is preferably a metal oxidelayer comprising aluminum oxide, tantalum oxide or other transitionmetal oxides such as titanium oxide, zirconium oxide, hafnium oxide,etc., or other material such as magnesium oxide. The thickness of theoverlayer 68 may range from approximately 10–80 Å. As so constructed,the overlayer 68 is thermally stable and will form a sharp non-diffuseinterface with the free layer 64 that promotes elastic scattering orspin-dependent reflection of sense current electrons with consequentmaintenance of GMR effect and with minimal shunting of sense currentaway from the ferromagnetic (and spacer) layers of the sensor.

The overlayer 68 can be formed using a suitable physical vapordeposition technique, such as ion beam deposition or magnetronsputtering. FIG. 9 illustrates an exemplary method for forming theoverlayer 68. According to this method, the free layer 64 is formed in aprocess step 80 using conventional techniques. In step 82, the overlayer68 is deposited on top of the free layer 64 using the desired physicalvapor deposition technique. By way of example, ion beam deposition maybe used with xenon (Xe) atoms to bombard at low pressure (e.g., 0.1mTorr) a metal target comprising the desired metal of the overlayer 68.The foregoing operation will be performed in the presence of a suitableoxidizing mixture introduced at a suitable mass flow rate to oxidize themetal target. An exemplary oxidizing mixture is 80% argon/20% oxygen. Anexemplary mass flow rate of this oxidizing mixture is 12–22 sccm(standard cubic centimeters per minute). The foregoing oxidationparameters have been found to minimize damage to the free layer 64 as aresult of excessive oxidation while maintaining sufficient oxidation toproduce an effective overlayer material.

Step 82 may also be performed using magnetron sputtering. In that case,a metal target comprising the desired metal of the overlayer 68 can bebombarded with argon (Ar) ions in the presence of a magnetic field atrelatively low pressure (e.g., <1–5 mTorr). This operation will beperformed in the presence of a suitable oxidizing mixture (e.g., 80%argon/20% oxygen) introduced at a suitable mass flow rate to oxidize themetal layer.

In step 84, the entire sensor stack structure is conventionally annealedto orient the exchange coupling between the AFM layer 58 and the pinnedlayer 56. Advantageously, it is believed that this annealing process mayresult in a reduction reaction in the free layer 64 that drives excessoxygen (caused by inadvertent oxidation thereof) into the overlayer 68,thereby tending to purify the free layer material. This may improve thequality of the interface between the metallic free layer 64 and theinsulative overlayer 68 relative to the spin dependent reflection ofsense current electrons.

Future spin valve sensors will require free layer magnetic thicknessesof 25 Å or below. Advantageously, the overlayer 68 permits controlledreduction of free layer magnetic thickness to these levels withoutreducing free layer physical thickness, thus allowing the sensor 50 topreserve high GMR ratio and achieve controlled negative free layermagnetostriction. These benefits are illustrated in FIGS. 10, 11 and 12,which show test results for two groups of sensors. A first group ofsensors was built according the invention, with the overlayer 68 beingformed from tantalum oxide deposited using ion beam deposition per theabove. The free layer magnetic thickness of each sample was varied bychanging the oxidation level of the overlayer 68 without altering thesample's free layer physical thickness. A second group of GMR sensorshad tantalum capped free layers. The free layer magnetic thickness ofeach sample was varied by altering the sample's free layer physicalthickness.

In FIG. 10, free layer magnetic thickness in angstroms is plotted forsensors in the first test group, with each sensor differing by theamount of sccm flow used during overlayer formation based on anoxidation mixture comprising 80% argon/20% oxygen. The material used forthe free layer of each sensor was CoFe and was deposited to a thicknessof 30 Å. The overlayer material (tantalum oxide) was deposited to athickness of 60 Å.

As shown by the curve 90 in FIG. 10, as the oxidation mixture (80%argon/20% oxygen) flow rate was varied from approximately 12 to 22 sccm,the free layer magnetic thickness was reduced from approximately 35 to27 Å, in substantially linear fashion, without varying the free layerphysical thickness. By extrapolation, it will be seen that magneticthicknesses of 25 Å or less could be achieved by using an 80% argon/20%oxygen flow rate of 24 sccm or greater.

As further shown in FIG. 11, the GMR ratio of the sensors in the firsttest group did not decrease as free layer magnetic thickness wasreduced. Instead, as shown by the curve 100 in FIG. 11, as the freelayer magnetic thickness decreased from approximately 35–26 Å, thesensor GMR ratio stayed relatively flat within a range of approximately14.5–15 Å. By extrapolation, it will be seen that free layer magneticthicknesses of 25 Å or less could also be achieved without significantlylowering GMR ratio or sensor sensitivity. This is contrary to thedecrease in GMR ratio observed in sensors of the second test group inwhich reduction of free layer magnetic thickness was achieved byreducing free layer physical thickness. The curve 102 in FIG. 11 showsthis conventional relationship.

FIG. 12 shows that a more desirable negative magnetostriction wasachieved for sensors in the first test group than for sensors in thesecond test group throughout the desired free layer thickness range. Inparticular, the curve 110 in FIG. 12 shows that as free layer magneticthickness was decreased from approximately 34–27 Å, the free layermagnetostriction of sensors in the first test group actually decreasedfrom slightly negative to approximately −2×10⁻⁶. By extrapolation, itwill be seen that free layer magnetostriction would become even morenegative at free layer magnetic thicknesses of 25 Å or below. This iscontrary to the undesirable increase in magnetostriction observed insensors from the second test group with conventionally reduced freelayer physical thickness, as shown by the curve 112 in FIG. 12.

Accordingly, a GMR spin valve sensor, a read head and a disk drive,together with methods for sensor fabrication, have been disclosed.Advantageously, the combination of reduced free layer magneticthickness, high GMR ratio and negative magnetostriction allows thefabrication of high-performance spin valve sensors with full benefitfrom the high excitation offered by sensor free layers that aremagnetically as thin as 25 Å or below. While various embodiments of theinvention have been described, it should be apparent that manyvariations and alternative embodiments could be implemented inaccordance with the invention. It is understood, therefore, that theinvention is not to be in any way limited except in accordance with thespirit of the appended claims and their equivalents.

1. A method of making a GMR sensor with reduced free layer magneticthickness without reduced free layer physical thickness comprising:forming a ferromagnetic pinned layer having a substantially fixedmagnetic moment; forming an electrically conductive spacer layer abovesaid ferromagnetic pinned layer; forming a ferromagnetic free layerabove said electrically conductive spacer layer such that said freelayer has a physical thickness (t) and a magnetic thickness (Mr*t)representing the product of a remanent magnetic moment density (Mr) ofsaid free layer and said free layer physical thickness (t); and reducingsaid free layer magnetic thickness by fanning a protective electricallynon-conducting overlayer on said free layer using a deposition processin which said overlayer is deposited directly on said free layer in thepresence of an oxidizing atmosphere that is controlled, to reduce saidfree layer magnetic thickness to a desired level without reducing saidfree layer physical thickness.
 2. A method in accordance with claim 1wherein said overlayer comprises a metal oxide material.
 3. A method inaccordance with claim 1 wherein said overlayer comprises a materialselected from the group consisting of aluminum oxide, tantalum oxide,titanium oxide, zirconium oxide, hafnium oxide and magnesium oxide.
 4. Amethod in accordance with claim 1 wherein formation of said overlayercomprises: deposition of said free layer; deposition of said overlayer,and annealing said sensor.
 5. A method in accordance with claim 4wherein deposition of said overlayer comprises depositing metal oxidematerial by physical vapor deposition in the presence of an oxidationgas mixture.
 6. A method in accordance with claim 1 wherein depositionof said overlayer comprises depositing metal oxide material by ion beamdeposition or magnetron sputtering in the presence of an argon oxygenoxidation gas mixture.
 7. A method in accordance with claim 1 whereindeposition of said overlayer comprises depositing metal oxide materialby ion beam deposition using xenon atoms with oxidation being providedby an Ar-20%O₂ gas mixture delivered at a flow rate of approximately12–22 sccm.
 8. A method in accordance with claim 1 wherein said freelayer magnetic thickness is in a range of approximately 26–35 Å.
 9. Amethod in accordance with claim 1 wherein said sensor has a GMR ratio ina range of approximately 3–15 percent.
 10. A method in accordance withclaim 1 wherein said free layer has negative magnetostriction in a rangeof approximately −2×10⁻⁶ to less than zero.